This is invention relates to the apparatus for drilling a hole at a precisely predetermined position at the intersection of two structural members and more particularly to a self-indexing apparatus that can position a drill at the precise center between two overlapping flanges of a wing stringer and rib and can clamp at that position while a self feeding drill drills the hole at the precisely predetermined position at the overlapping flanges of the two structural members.
Airplane wings are built by fastening a stringer, which is a Z-shaped or U-shaped structural member, to the wing skin, and then fastening the ribs which run laterally across the span of the wing onto the stringers. The ribs, in turn, are fastened to longitudinally extending wing spars. The connection of the wing ribs to the stringers is done by drilling a hole through the intersections of the stringer flanges and the horizontal leg of the rib. Fasteners are inserted in the holes and are tightened to provide a secure attachment at every intersection of the ribs to securely fasten the wing skin to the wing frame.
The placement of the hole at the intersection of the rib and the stringer is important for the achievement of maximum strength and to insure that the fastener heads are not in contact with the rib flange which might cause a fatigue problem with the fastener. In the past, placement of the hole at the intersection has been achieved by clamping a drill guide which in place and using it to guide a hand held drill to drill a hole through the overlapping flanges. This method is generally acceptable and always produces accurate results unless the drill guide is improperly placed or slips after being properly placed. Also, if the wrong drill guide is used, or the guide becomes damaged so that it no longer properly positions the hole at the right location the hole may not be properly positioned. To guard against occurrences of this nature, the holes are routinely checked by quality control to insure that they are properly placed. Two instruments for measuring the position of the hole at the intersection of a flange and a rib are disclosed in U.S. patent application Ser. No. 555,869 filed on Jul. 20, 1990.
The procedures to repair an improperly drilled hole are time consuming and expensive, and the repair adds weight to the airplane which reduces its operating efficiency. Therefore it is far preferable to insure that the hole is drilled at the correct location in the first instance so that no repair is necessary.
Accordingly, there has long been a need in the airplane manufacturing art for an apparatus which will correctly position and accurately drill a hole at the intersection of a rib and a stringer, which can be used in most of the locations in the wing. The apparatus should be easy and fast to operate and should clamp securely in position to eliminate the possibility of inadvertent movement from the correct position.